One of the officials said it had the look and consistency of cornstarch. In addition to the powder, the envelopes contained a letter referring to al-Qaida and the FBI, that official said. The officials spoke on condition of anonymity because they were not authorized to release the information.
No one was injured by the powder.
The letters were sent from out of state to about 25 schools, but James McJunkin, the head of the FBI's Washington field office, declined to be more specific. WRC-TV in Washington obtained an image of one of the letters that had a Dallas postmark. The stamp appeared to be canceled on May 2, the day after the U.S. announced it had killed al-Qaida leader Osama bin Laden.
McJunkin said the addresses on the letters were printed, not handwritten. Each letter was addressed to a school and not a specific person.
He would not say where the other schools around the country that had received similar letters were located.
He said the substance likely was not harmful, but after preliminary testing would be sent to the FBI's Quantico facility, where the letters would undergo a full forensic screening. However, the substance found in the similar letters mailed elsewhere turned out to be harmless, he said.
Mayor Vincent Gray condemned the letters.
"I think it's a dastardly act," he said. "It alarms people unnecessarily."
People have been wary of powdery substances in letters since the 2001 anthrax scare. The government eventually determined that Bruce Ivins, a researcher who worked at Fort Detrick in Maryland and later committed suicide, was behind the mailings of powdered spores. In a nation still reeling from the Sept. 11 attacks, five people died in October and November that year from anthrax inhalation or exposure linked to the letters.
District schools started reporting receiving the letters about 1 p.m. Thursday.
Schools will open normally Friday, said Metropolitan Police Department Chief Cathy Lanier.
She said police were working with U.S. postal inspectors to make sure that mail delivered to schools is safe, but she declined to be specific about any plans for screening the mail.